1. Field of the Invention
The present invention relates to lab based over-the-air (OTA) testing of multiple antenna devices, and more particularly to techniques for OTA testing suitable for producing a test signal to emulate a wireless channel while using a limited number of transmit elements.
2. Description of Related Art
When designing over-the-air (OTA) testing approaches, channel modeling concepts are applied in order to insure that the received signal is representative of the desired test condition. Modern radio systems use different technologies including spread spectrum techniques such as wide-band code division multiple access (WCDMA), and orthogonal frequency division multiple access (OFDMA). These two technologies are significantly different in the way they process the receive signal, leading to differences in how the RF channel is modeled. This impacts the design of an OTA test.
For wide band radio systems, the multi-path radio channel can be modeled as a series delayed copies of the signal. For spread spectrum systems, such as WCDMA, each delayed copy of the signal that is resolvable within the radio bandwidth is called a path, and is typically characterized by a narrow angle spread. Each path is detected and may be processed or combined in a manner determined by the design of the receiver. Thus for this type of air interface, the paths are carefully specified to allow multiple antenna processing to be correctly emulated.
OFDM systems rely on the data being encoded and transmitted in a series of narrow band sub-carriers spaced across the operating bandwidth of the channel. As such, the signals are inherently narrow band, and not able to distinguish the individual paths that are modeled for the spread spectrum systems. Rather, the channel is modeled as the composite of all signal paths. Thus the spatial channel effects will be observed using all paths in combination, and typically result in higher angle spread values. The wide-band effects of the multi-path channel will be observed as frequency selective fading across the band affecting each sub-carrier, although the individual paths will not be distinguishable.
Use of predefined channel models can be used to define an OTA test. Recorded signals obtained from channel measurements can also be used in a play-back fashion to generate test signals.
Most channel models such as the Spatial Channel Model (SCM) have been designed to be antenna independent, i.e. they are suitable for a variety of possible antenna arrangements. Thus the model provides a way to specify a stochastic geometric based spatio-temporal description of the paths between a base station (BS) and a mobile station (MS), such that the performance of the antennas of the base station and mobile station can be applied and modeled separately from the channel model. The base station and the mobile station will then make use of the paths in the channel model along with their antenna characteristics to produce a complete end-to-end channel for evaluation and testing.
In order to include the antenna characteristics in a measurement of radio link performance using an OTA technique, it is necessary to generate and wirelessly transmit an accurate test signal such that the antennas of the device-under-test can act upon the received signal in a controlled way. For the purposes of testing, a signal to be received by the MS can be generated without requiring the BS to be present in the simulated path, or visa-versa. This is done by providing the proper signals to the transmitting and receiving elements of a portion of the channel that is being modeled.
However, generating the test signals for an over-the-air test is complex. There are multiple paths, and each path must be constructed to produce the proper angle of arrival and angle spread, to produce the proper correlation between antennas at the device-under-test. Thus many individual transmit elements (also referred to as probes herein) would typically be required to be located in a variety of positions with an anechoic chamber to produce an adequate test signal. Having many probes is costly and complex to implement. Furthermore, having many probes will degrade the characteristics of the chamber due to additional reflections that may be produced.
Therefore it is desirable to techniques for OTA testing suitable for producing a test signal to emulate a wireless channel suitable for evaluating multiple antenna devices while using a limited number of transmit elements.